D. V. Efremov

Superfluid transition temperature in a Fermi gas with repulsion allowing for higher orders of perturbation theory

Higher-order perturbation-theory corrections to the superfluid transition temperature in a weakly nonideal Fermi gas with repulsion are determined. This involves calculating the contribution of third-and fourth-order diagrams in terms of the gas parameter apF to the effective interaction which determines the super-fluid transition temperature and also allowing for effects associated with retardation and renormalization of the polar part of the Green’s function. The expressions obtained provide evidence in support of attraction in the effective interaction in the second, third, and fourth orders of perturbation theory. It is shown that the critical temperature is mainly determined by second-and third-order terms of perturbation theory. Calculations are made of the superfluid transition temperature for a gas comprising neutral Fermi particles in a magnetic field. The limits of validity of the theory are analyzed and the possibility of applying the results to dilute solutions of 3He in 4He and neutral-particle Fermi gases in magnetic traps is discussed.

The enhancement of the superconductive transition temperature in quasi-2D materials in a parallel magnetic field

Abstract It is shown that a strong enhancement of the superconductive transition temperature is possible in quasi-2D materials when a high parallel magnetic field is applied. Under these circumstances the intralayer triplet pairing of the BCS-type takes place. This pairing originates from the very strong 2D Kohn anomaly in the effective interaction between two electrons in one layer through the polarization of the electrons of the same layer. The possibilities of the experimental observation of this phenomena in real substances are discussed.

Critical temperature and phase diagram for a 2D superfluid Fermi gas with repulsion

Abstract In the framework of a 2D Fermi gas with short-range repulsive interaction we find the critical temperature of the superfluid phase transition based on Kohn–Luttinger effect, and analyze its dependence on magnetic field. We calculate strong coupling corrections to the Ginzburg–Landau free energy functional and analyze the stability of 2D superfluid phases. Possible applications of the results to real systems are discussed.

Phase diagram for the superfluid fermigas in a strong magnetic field

We calculate the strong-coupling corrections to the Ginzburg-Landau free-energy functional in a strong magnetic field and analyze local and global minima of this functional. We show that the global minima in strong magnetic field correspond in the case of Fermi-gas with repulsion only toA1 andA2 phases discovered by Anderson, Brinkman and Morel [1].

Two-particle pairing in 2D Bose gases

Abstract We consider a possibility of two-boson pairing in a dilute 2D Bose-gas with strong hard-core repulsion and a Van der Waals attractive tail. We show that the phase diagram for a dilute Bose-gas of one sort of structureless bosons consists of two regions. The first one is the region of usual one-particle Bose–Einstein condensation. But the second one is the region of total phase separation on Mott–Hubbard Bose-solid and dilute Bose gas. However already for Bose-gas consisting of two sorts of structureless bosons described by the two band Hubbard model the s-wave pairing of the two bosons of different sorts 〈 b 1 b 2 〉=0 is possible.

Kohn-Luttinger effect and anomalous pairing in repulsive Fermi-systems at low density (Review Article)

We demonstrate the possibility of triplet p-wave pairing at low electron density a large number of models such as 3D and 2D Fermi-gas models with hard-core repulsion, 3D and 2D Hubbard models, and the Shubin-Vonsovsky model. The critical temperature for p-wave pairing can be considerably higher in the spin-polarized case or even in a two-band situation at low density and can reach experimentally observable values of 1–5u2009K. We also discuss briefly the d-wave pairing and high-Tc superconductivity with Tcu2009∼u2009100u2009K which arise in the extended Hubbard model and in the generalized t-J model when close to half-filling.

Superfluidity in Fermi-Systems With Repulsion

We discuss the superfluid transition in Fermi-system with repulsive interaction. In the model of weakly interacting Fermi gas with short range repulsion the presence of Kohn singularity in the effective interaction leads to the Cooper pairing in the p-wave channel. We show that the superfluid critical temperature in spin-polarized system exhibits a strong nonmonotonic behavior as a function of polarization. We also study the superfluid p-wave phase diagram for a 2D Fermi-liquid. In the weak-coupling limit the absolute minimum corresponds both to planar and axial phases. To lift this degeneracy, we calculate the strong- coupling corrections (i. e. next order of T C/eF) to the quartic terms and find the most possible candidates to the phase stabilized in the system.

Superfluidity and superconductivity in fermionic systems with repulsion

Abstract The critical temperature of a superfluid phase transition in a Fermi gas with repulsive interaction is found. The influence of a magnetic field on the transition is analyzed. The estimates for the critical temperature for a trapped gas of 6 Li atoms and 3 He– 4 He mixtures are presented.

SUPERCONDUCTIVITY IN CORRELATED ELECTRON SYSTEMS WITH REPULSION

Abstract On the basis of the ideas of Kohn and Luttinger we study superconductivity in purely repulsive Fermi-systems at low density. We obtain superconductive critical temperatures in different 3D and 2D models. These results are important in connection with an experimental search of superconductivity in simple and noble metals, heterostructures and superlattices, together with the layered compounds such as dichalcogenides and Sr 2 RuO 4 . We provide an elegant way to increase T c already at low density in the presence of a strong magnetic field or in a two-band situation.